This invention relates to a superconducting switch and a current limiter for electromagnetically limiting an overcurrent in an a.c. electric path using such a superconducting switch.
When an overcurrent flows in electric equipment, it is required for protecting the electric equipment from such an overcurrent to limit flowing of an overcurrent the moment it occurs. For a current limiter for limiting an overcurrent, there is known a current limiter disclosed in, e.g., the Japanese Patent Publication Laid Open No. 74932/85. In this current limiter, two coils are wound on an iron core so that their magnetomotive forces are substantially equal to each other. Respective ends of both coils are connected to an electric path on the side of the power supply so that the directions of their magnetic flux are opposite each other. The other end of the coil on one side is connected to an electric path on and load side through a switch. Furthermore, the other end of the coil on the other side is similarly connected to the electric path on the load side. A current limiting resistor is connected in parallel with the switch. In addition, a current transformer is provided in the electric path. This current transformer serves to trip the switch when it detects an overcurrent.
In this circuit, when an overcurrent flows in the electric path due to short-circuit of the load, the current transformer detects this phenomenon to open the switch to insert the current limiting resistor into the circuit of the coil on one side. Thus, while a current flowing in the coil on one side is decreased, a current flowing in the coil on the other side is increased. As a result, the magnetic flux produced by the coil on the other side wound on the iron core is dominant as compared to that produced by the coil on one side wound thereon. Accordingly, the inductance of the coil on the other side becomes active, i.e., a failure current is limited by the action of reactor.
Since a current of several hundreds to several thousands of amperes exists in an ordinary state in the above-described current limiter, both coils are required to have a large cross section, and they are also required to have an increased number of turns in order to provide a large current limiting impedance. This results in the problems that the current limiter become large-sized, and that a large amount of power loss due to heating cannot be avoided.
Furthermore, the above-described current limiter often uses a mechanical switch, and therefore requires a time of one to three cycles from the time when an overcurrent is detected until the switch is opened to carry out the current limiting operation, resulting in the problem that the electric path is difficult to protect.
For this countermeasure, a semiconductor switch such as a thyristor, etc. may be used. In this case, a power loss is produced by a voltage drop in the forward direction of the thyristor. Consequently, the current limiter further becomes large-sized and complicated, with the result that the employment of such a semiconductor switch was difficult.
For this reason, it has been proposed to use a current limiting body utilizing a superconductor. Namely, a current limiting element made up by forming a superconductive body in the form of a rectangular wave is connected in series with a circuit including a power source, an interrupter, a line impedance, and a load. In the circuit thus constructed, when a current i.sub.o flows in the load via the current limiting body, the current limiting body in a superconducting state. The value of a current flowing in the current limiting body is in a range smaller than a critical current value J.sub.cl. Assuming now that a critical current J.sub.cl flows in the current limiting body, the current limiting body produces quenching to rapidly shift to a normal conducting state. At this time, the resistance of the current limiting body abruptly increases to its intrinsic resistance value. By this high resistance value, a current flowing in the load through the current limiting body is limited. The resistance value R of the current limiting body when quenching is produced in the current limiting body as stated above is expressed as R=.rho.c (1/A). Namely, the resistance value R of the current limiting body is proportional to product of the intrinsic resistance .rho.c (.mu..OMEGA.-cm) and the length l (cm) of the current limiting body, and is inversely proportional to cross section A (cm.sup.2) of the current limiting body.
However, while the critical current density of well known Nb-Ti based superconductor has a very large value of the order of 1 to 3 (KA/cm.sup.2), the intrinsic resistance at a normal conducting time has a very small value of the order of 20 to 50 (.mu..OMEGA.-cm). Accordingly, only in the case of a superconductor in the form of a rectangular waveform, is the cross section A (cm.sup.2) is large as well as the length is not sufficient, so that a sufficiently high resistance value at the time of quenching of the superconductor cannot be obtained. For this reason, a coil-shaped superconductor which is further thinned and elongated may be devised in order to provide a high resistance value. However, since such a coil-shaped superconductor has an inductance, an impedance voltage drop would occur with respect to a steady state current and a high surge voltage at the time of quenching is produced, exerting an adverse influence on the circuit equipment. Furthermore, limiting of current is not definitely carried out by a delay based on the inductive component. In addition, since a high voltage is applied to a superconductor at the time of quenching, there is the problem that the dimension between coils must be large in order to obtain a sufficient withstand voltage.